This invention relates to a focusing method and apparatus suitable for use in a scanning electron microscope or a charged particle beam apparatus using a charged particle beam for dimension measurement of LSI's, IC's, etc.
The technical task of automatic focusing in a scanning electron microscope or the like has been how attainment of focusing can be detected with high accuracy.
A method commonly employed hitherto for detecting the state of focusing utilizes the fact that the frequency of a picture image signal from a scanning electron microscope or the like becomes progressively high with the decrease in the spot diameter of the charged particle beam. According to the prior art method, weight coefficients proportional to frequencies are used for multiplication, and the results of multiplication are summed for comparison of output signals. The method described above is generally called a differentiated value comparison method since a differentiator is incorporated in the detection circuit. (Refer to Japanese Patent Application Laid-open No. 5642945 (1981), columns 3 to 5.) However, the prior art method has been defective in that various noises in pulse form appear due to the use of a photomultiplier tube and an operational amplifier in the detection system of the scanning electron microscope, and, especially, in the case of a specimen having a bad S/N ratio (especially, a semiconductor specimen such as an IC or an LSI), the detection circuit tends to mal-operate by being adversely affected by such noises, resulting in impossibility of attainment of automatic focusing with high accuracy.